This refactoring is preparatory work to enable ICU to be the backend
for timezone information rather than system calls. In the process, a
bit of code duplication that was inserted in the Solaris port patch is
eliminated here among modern POSIX backends.

One possible performance downside of this patch is that it introduces
a virtual method call for operations which were previously not virtual
methods. However, a couple factors mitigate this effect:
- The DateCache minimizes the need for calls into the TimezoneCache
- These calls were already not very high performance, as they included
  a system call which requires an RPC to get out of the sandbox, and
  they are surrounded by C++ builtins, which require a JS to C++
  transition.
- A future transition to ICU, enabled by this refactoring, may improve
  performance by eliminating the system call.

BUG=v8:6031

Review-Url: https://codereview.chromium.org/2731463003
Cr-Commit-Position: refs/heads/master@{#43588}